Patients with early and moderately advanced mycosis fungoides (MF) are treated topically with solutions of 3 nitrosourea (NU) compounds in a continuing evaluation of the efficacy and safety of such therapy. To date 26 such patients have been treated topically with 3 NU compounds: 1,3-bis(2-chloroethyl)-1-NU (BCNU), 1-(2-chloroethyl)-3-cyclohexyl-1-NU (CCNU), and 1-methyl-1-NU (MNU). Good to excellent results have been obtained in a high percentage of cases. Patients allergic to topical mechlorethamine can be treated with NU compounds without danger of cross-sensitivity reactions. Bone marrow depression occurred in 2 patients and was attributed to CCNU. No hematopoietic or other toxicity was noted in patients treated with BCNU alone, and BCNU is presently the preferred NU compound in the treatment of MF. The percutaneous absorption of BCNU is in the range of 20-30%. Results of long-term painting of mice with NU compounds indicate that BCNU and CCNU are weak topical carcinogens, whereas MNU is a strong topical carcinogen.